In 2006, Breen was brought on to develop the first season of the ABCdramaBrothers & Sisters, as a writer and supervising producer. She then went on to become co-executive producer at the beginning of season two, and then held the position of executive producer throughout seasons three and four. At the conclusion of the fourth season, Breen left the series. The eleven episodes she co-wrote are as follows:

In Fall 2006, Breen joined the third season crew of ABC's Lost, as a writer and supervising producer. She left the series in February 2007. She co-wrote one episode while working on the series, with supervising producer Alison Schapker, "The Cost of Living".